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The Effect of Acidic and Alkaline Chemical Solutions on the Behavior of Collapsible Soils

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Cite this article as: Khodabandeh, M. A., Nokande, S., Besharatinezhad, A., Sadeghi, B., Hosseini, S. M. "The Effect of Acidic and Alkaline Chemical Solutions on the Behavior of Collapsible Soils", Periodica Polytechnica Civil Engineering, 64(3), pp. 939–950, 2020. https://doi.org/10.3311/PPci.15643

The Effect of Acidic and Alkaline Chemical Solutions on the Behavior of Collapsible Soils

Mohammad Ali Khodabandeh1*, Saber Nokande2, Ali Besharatinezhad1, Behnam Sadeghi3, Seyed Mahdi Hosseini4

1 Department of Engineering Geology and Geotechnics, Faculty of Civil Engineering, Budapest University of Technology and Economics, H-1111 Budapest, Műegyetem rkp. 3., Hungary

2 Department of Geotechnical Engineering, Faculty of Civil Engineering, Semnan University, 19111-35131 Semnan, Iran

3 Department of Environmental Engineering, Faculty of Civil Engineering, College of Environment, Karaj 31746-1, Iran

4 Department of Geotechnical Engineering, Faculty of Civil Engineering, Shahrood University of Technology, 3619995161 Shahrood, Iran

* Corresponding author, e-mail: mohammad.khodabandeh@epito.bme.hu

Received: 26 January 2020, Accepted: 02 May 2020, Published online: 16 July 2020

Abstract

In this research, the effect of acidic and alkaline chemical solutions on the behavior of loessial soil was investigated. To evaluate the severity of acidity and alkalinity of chemicals, two factors sulfuric acid and sodium hydroxide were used in the pH of 3, 5, 9, 11.

In this research, the effect of acidic and alkaline solutions on the collapse potential, shear strength parameters and unconfined compression of collapsible soils were investigated. Experimental tests results showed that acidic solutions with a low pH increase the collapse potential and effective cohesion of soil and decrease effective internal friction angle; on the other hand, alkaline solutions with a high pH decrease the collapse potential and effective cohesion of the soil and increase effective internal friction angle. The results of unconfined compression tests showed that with increasing the acidity and alkalinity in soil, the undrained strength of the soil decreased. SEM test results showed an increase in soil cavities in acidic solution while the soil cavities were fixed in alkaline solution.

Keywords

collapsible soil, collapse potential, shear strength, acid, alkali

1 Introduction

The occurrence of loess deposits is worldwide and can be found extensively from South America [1] to Africa [2], Europe [3], Asia [4–6], and Oceania [7]. In summary, about 10 % of the Earth's surface has been covered by this special type of geomaterial [8]. The foundations con- structed on collapsible soils might settle due to the satu- ration of the soils for various reasons such as water pipe failure, sewage leakage, drainage of reservoirs or pools, rising groundwater level, leaks of industrial effluents and chemicals, etc. [9]. Several studies have been conducted on the effects of leachate and chemicals solutions on the behavior of granular and fine-grained soils. A point is that in all previous research, the results were different, for each type of soil and no coherence was observed between the outcomes of the studies. Therefore, further consideration is required to understand the behavior of collapsible soils when they are subjected to chemical attacks.

The previous works were focused on soil type, stress condition, permeability, and the acid and alkali type.

Kaya and Fang [10] studied the effect of organic fluids on kaolinite and bentonite soils and concluded that when void space in soil is replaced with organic liquids, soil engi- neering properties, such as hydraulic conductivity and stress-strain behavior were significantly altered.

Ratnaweera and Meegoda [11] examined the uncon- fined compressive strength of fine-grained soils contam- inated with various types of chemicals (Glycerol, propa- nol and acetone). The results showed a decrease in shear strength and stress-strain behavior of contaminated soils.

This decrease in shear strength is attributed to the phys- ico-chemical effects created by the decrease in dielec- tric constant and the mechanical interactions induced by high pore fluid viscosities. In another study by Wang and Siu [12], the structural and mechanical properties

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of kaolinite when faced with different pH levels were investigated. They found that acidic liquid increased the compression of soil due to the reduction of void space.

Sridharan et al. [13] stated that the shear strength behavior of kaolinite under the influence of organic fluids differed greatly in comparison with montmorillonite. It is notable that, while the undrained shear strength value increased in kaolinite, it decreased in montmorillonite.

Moavenian and Yasrobi [14] examined the effect of organic liquids on volume change behavior of clayey soils.

Results showed that pure organic chemicals caused less heaving in comparison with distilled water. Also, contam- inated soil’s plasticity reduced, and organic fluid caused osmotic consolidation in soil. Tang et al. [15] found that anionic surfactant would weaken the loess properties, which could lead to a decline in cohesion, improved com- pressibility, and decreased permeability, but can reduce the collapsibility of loess.

Olgun and Yildiz [16] observed that with increasing the acetic acid (20 %, 40 %, 60 %, 80 %) in clay soils, the cohesion of soils increased. Also, they observed that the liquid limit and plasticity index have been significantly decreased in montmorillonite due to the collapse of the diffuse double layer and in kaolinite increased slightly due to gelification. Spagnoli et al. [17] represented that when the pH value of solutions changes, the undrained shear strength of the kaolinite and montmorillonite mixed clay increases. Thus, it can be noted that the chemical property of the pore water is an important factor in the shear strength of soil. In another study, Yang et al. [18] concluded that the acidic conditions caused the erosion of cementitious soils, while alkaline conditions had less effect on soil properties.

They observed that in acidic and alkaline conditions the unconfined compressive strength (UCS) of cementitious soils decreased (around 30 % of initial strength).

Rahman and Nahar [19] investigated the effect of pH on shear strength behavior of granular soil and concluded that the shear strength increase with the increase of pH values of soil. Reddy et al. [20] have examined the effect of acidic and alkaline fluids on expansive soil fluctuation behavior. The results showed that swelling of soil contam- inated with sodium hydroxide first increased in lower con- centrations and then decreased with increasing concen- tration; however, the swelling of soil contaminated with sulfuric acid decreased at low concentration, and then increased with increasing concentration of the sulfuric acid solution.

Li et al. [21] have investigated the effect of alkaline leachate on the mechanical properties of stiff clays (CL) and they observed a decrease in cohesion and an increase in the internal friction angle with increasing the alkaline leachate (pH > 7) in the soil. Sunil et al. [22] have investi- gated the effect of alkaline leachate on SC coarse-grained soils and they found an increase in cohesion and a decrease in the internal friction angle of the soil with an increase in alkalinity of the soil.

Overall, the acidic and alkaline chemical solutions could significantly change the physical and mechan- ical properties of the soils. Although most of the previ- ous studies have focused on clay and sandy soils, fewer studies were conducted on the problematic soils including collapsible soil. In this study, the changes in physical and mechanical properties of collapsible soils when they con- taminated with chemicals at a different range of pH levels were investigated.

2 Materials 2.1 Soil

The soil used in this study was loess which was taken from Kalaleh area in Golestan province, Iran, where sampling coordinate is (37°30'12.6"N 55°30'42.9"E). Fig. 1 shows the location of the sampling site. Undisturbed soil samples were taken in 30 × 30 × 30 cm3 boxes from 1 m below the ground surface to prevent the upper organic soil layer and grass roots. Immediately after sampling, the boxes were isolated with paraffin to prevent the loss of moisture.

The clay soil was classified as CL according to the unified soil classification system (USCS) category. The particle size distribution of the soil is presented in Fig. 2.

Fig. 1 The location of the sampling site

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The Atterberg limit test has been carried out on the soil according to the ASTM D4318-87 standard, and the results of liquid limit and plasticity limits are reported in Table 1. The specific gravity (Gs) and the pH of the soil were determined according to the ASTM 854-92 standard and ASTM D4972-01 standard, respectively. The result is shown in Table 1.

2.2 Chemicals

In order to simulate the effect of chemical solutions on the behavior of collapsible soils, sulfuric acid and sodium hydroxide were used to prepare solutions with different pH levels. Note that the required amount of sulfuric acid and sodium hydroxide was chosen to reach the pH levels of 3, 5, 7, 9 and 11 to simulate the most critical condition.

The purity of sulfuric acid and sodium hydroxides were 98

% and 95–98 %, respectively.

3 Laboratory program 3.1 Sample preparation

Tests were conducted in undisturbed and remolded con- ditions. Each type of sample was used for a specific pur- pose; undisturbed samples were used to test soils in their natural skeleton, while remolded samples were used to make homogeneous mixtures of soil and contamination.

Remolded samples were made with the same void ratio and unit weight as those of undisturbed samples. To construct remolded samples, portions of the box (30 × 30 × 30 cm3) samples were completely crushed, dried in the oven, passed from a No. 40 sieve, and then uniformly mixed with de-aired distilled water at an average moisture con- tent of 3.5 % (almost equal to the mean field natural value).

Then, for a more uniform distribution of water content the mix was placed in a sealed plastic bag for 48 hours.

After that, the soil was used for different tests. Remolded samples were completely saturated with chemical solu- tions with different pH level ranges to assess contamina- tion in the saturated state to determine the collapsibility and shear strength parameters of collapsible soils. Also, other remolded specimens were contaminated with chem- ical solutions at 5,10 and 15 % by weight of the dry soil samples for the UCS test. This form of sampling was cho- sen because there is no possibility to saturate the samples during the UCS test. Then, the samples were placed in insulated plastic containers for 20 days for aging, achiev- ing an equilibrium state and allowing possible reactions between soil and chemicals.

3.2 Collapse test

The measurement of the collapse potential of soil was con- ducted according to the ASTM D5333 standard. The col- lapse test is similar to the consolidation test since the spec- imen is internally consolidated. After placing the sample in the device and closing the measuring equipment, the ini- tial stress of 5 kPa was applied to the sample. After apply- ing initial stress, different levels of stress including 12.5, 25, 50, 100, 200 kPa were applied to the samples and the deformation (settlement) of soil was measured in each step.

Then the sample was wetted 1 hr after applying 200 kPa and the loading last for up to 24 hours in saturated condi- tion and the settlement was monitored. It should be noted that if the fluid is applied from the bottom of the sample, all the trap air comes out and the sample reaches to fully satu- rated condition. According to the test, the index of collapse which indicates the settlement of sample under the stress of 200 kPa, is obtained from Eq. (1) [23].

IC =(dfdi) /h0×100 (1) IC: the potential collapse

df: measurement the settlement after saturation di: measurement the settlement before saturation h0: initial height of the sample

Fig. 2 Grain size distributions of loess soil samples

Table 1 Soil property and value

Soil Collapsible soil

USCS CL

pH 7.9

GS 2.67

Natural water Content (%) 3.57

Ɣdry (gr/cm3) 1.42

LL (%) 29

PL (%) 21

PI (%) 8

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3.3 Setup for collapse test The plan of the test is as follows:

First, the collapse test was conducted on undisturbed soils to measure the amount of collapse. Then, the col- lapse test was performed on the remolded samples with the same density and moisture of the field. In this case, the chemical solutions which were made in the laboratory were used as fluids instead of water to determine the effect of pH on the collapse potential of soil.

It should be noted that the remolded samples were used for the study since the undisturbed samples have differ- ent initial moisture, dry density, and void index; therefore, it was hard to investigate the effect of pH on the collapse potential of soil when the others parameters are not same.

Thus, all the other parameters affecting the collapse rate were kept the same. Table 2 presents the program of col- lapse tests.

3.4 Direct shear test

In this study, the direct shear tests were performed on soil samples under drained conditions according to ASTM D3080/D 3080M [24]. The straight-sectional cube with a dimension of 60 × 60 × 60 mm was used to perform a direct shear test. The speed of upper shear box displace- ment was set at 0.048 mm/min for all the soil samples. The plan of the direct shear test is as follows:

First, the direct shear test was performed on the remolded soil with density and moisture of the field to estimate the shear strength of the soil; In this case, the samples were directly subjected to distilled water at the pH of 7 to saturate. Then, the direct shear test was per- formed on the remolded soil with a density and moisture of the field in condition that the saturation was happened by the chemical solution (acid sulfuric and water, sodium

hydroxide and water); In this case, to saturate the sam- ple, the chemical solutions which were made in the lab- oratory were used at different pH levels instead of water to determine the effect of acidic and alkaline solutions on the shear strength parameters of soils. Table 3 presents the program of direct shear tests.

3.5 Unconfined compression test

To evaluate the effect of acidic and alkaline chemical solu- tions on the undrained shear strength and stress-strain characteristics of collapsible soils, unconfined compres- sion tests were performed according to ASTM-D2166 standard [25] on the clean soil samples and also on the contaminated soil. The procedure was as follows: the soil samples were contaminated by acidic (pH = 3) and alka- line (pH = 11) solutions at the 5 %, 10 % and 15 % (by weight %), then after the treatment, unconfined compres- sion test was conducted on the clean and contaminated soil samples in unsaturated state. Table 4 presents the program of unconfined compression strength tests.

Table 2 The program of collapse tests Method

Used Sample

condition Saturating fluid Chemical solution content (%)

Collapse test (ASTM D5333)

undisturbed Distilled water

(pH = 7) 100

remolded Distilled water

(pH = 7) 100

remolded Acid sulfuric and

water (pH = 3) 100

remolded Acid sulfuric and

water (pH = 5) 100

remolded Sodium hydroxide

and water (pH = 9) 100 remolded Sodium hydroxide

and water (pH = 11) 100

Table 3 The program of direct shear tests Method

Used Sample

condition Saturating fluid Chemical solution content (%)

Direct shear test(ASTM D)

remolded Acid sulfuric and

water (pH = 3) 100

remolded Acid sulfuric and

water (pH = 5) 100

remolded Distilled water

(pH = 7) 100

remolded Sodium hydroxide

and water (pH = 9) 100 remolded Sodium hydroxide

and water (pH = 11) 100

Table 4 The program of unconfined compression strength tests Method

Used Sample

condition Contaminant type Chemical solution content (%)

Unconfined compression strength

Remolded Clean 0

Remolded Acid sulfuric and

water (pH = 3) 5

Remolded Acid sulfuric and

water (pH = 3) 10

Remolded Acid sulfuric and

water (pH = 3) 15

Remolded Sodium hydroxide

and water (pH = 11) 5 Remolded Sodium hydroxide

and water (pH = 11) 10 Remolded Sodium hydroxide

and water (pH = 11) 15

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4 Results

4.1 The results of collapse potential of undisturbed and remolded soils when saturated with water

To prove the collapse potential of soil, the collapse experi- ment was carried out to determine the collapse potential on remolded and undisturbed soils when both samples were sat- urated with water. The test results are presented in Table 5.

According to the results, the tested soil is placed in a highly collapsible category based on ASTM D5333 standard.

As can be seen in Table 5, the remolded samples condi- tion decreased the collapse rate, with the average collapse reduced from 17.85 % to 11.67 % for the remolded sample.

The reason for this reduction can be attributed to the void space and changes in soil structure which play a very effec- tive role in the collapse condition. As a result, it can be concluded that the disturb condition would reduce the void ratio and thus decrease the collapse potential of soil. Fig. 3 shows the comparison between the collapse behavior of both remolded and undisturbed samples.

As shown in Fig. 3, the collapse potential of soil reduced by 6 % in remolded samples, but still, the soil is in severe collapse condition. Fig. 4 shows the macroscopic and micro- scopic pores of the collapsible soil structure. Although disturbance may break down the macroscopic and larger pores, but there are also smaller pores that remolded effort can't break down them and remained collapse potential is caused by these pores.

4.2 The results of collapse potential of soil when saturated with chemical solutions at different pH levels It was observed in Fig. 5 that decreasing the pH of the solution (acidic condition) caused an increase in soil col- lapse. The amount of soil collapse was 11.67 %, 14.53 % and 16.75 % at the pH of 7, 5 and 3 respectively, which proved the acidic condition increased the collapse poten- tial of soil. Moreover, it can be seen that the amount of soil collapse was reduced from 11.67 % to 9.63 % when the pH of the solution increased from 7 to 11, respectively.

According to Fig. 5, it can be seen that the changes in the collapse potential of soil in acidic conditions (pH < 7) were more variable compared to alkaline conditions (pH > 7). In other words, in the most acidic condition, an increase of 5.08 % of the collapse was observed; however, in alkaline condition, only 2.04 % of change was observed in the collapse potential of soil.

When sulfuric acid is diluted in water, its breakdown occurs in two steps which are presented in Eqs. (2) and (3):

H SO + H O2 4 2 →H O + HSO3 + 4, (2) HSO4+ H O2 →H O + SO3 + 4-2. (3)

Table 5 Results of collapse tests for undisturbed and remolded soils in saturated condition with water Sample

conditions sample Initial void index Final void

index Initial

Moisture (%) Final

Moisture (%) γd (g/cm3) Rate of

collapse Average of the

rate of collapse Collapse condition

Undisturbed

1 1.05 0.37 3.72 18.87 1.3 19.58

17.85 severe

2 0.89 0.33 3.79 18.74 1.41 17.94

3 0.74 0.26 3.14 16.29 1.53 16.04

Remolded

4 0.9 0.48 3.5 20.43 1.42 12.5

11.67 severe

5 0.9 0.52 3.5 21.1 1.42 11.9

6 0.9 0.49 3.5 20.75 1.42 10.63

Fig. 3 Comparison of remolded and undisturbed samples collapse behavior

Fig. 4 SEM images of undisturbed loess soil: a) macroscopic image at 100x magnification, and b) microscopic image at 1000x magnification [26]

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First, sulfuric acid in combination with water decom- posed to hydronium (H3O) and ion sulfate. Hydronium ions, due to its smaller size and ionic potential (hydrated cation charge), could easily penetrates mineral crystals and leads to a decrease in the hydrogen bond between the successive units of kaolinite. Hydrogen ions can elim- inate iron in kaolinite networks (structure) and can cre- ate a new mineral by combining anions (SO4–2). However, SO4–2 anion remains in action and acts as an anion for cat- ions. Therefore, these reactions cause significant changes in soil microstructure. Moreover, H3O+ caused changes in the cation exchange in clay components, which leads to a change in the mineral content and soil microstructure [27].

Therefore, the reason for the increase in collapse soil in the acid-saturated state could be related to the destruction of the clay microstructure due to the interaction of clay minerals when faced with acidic fluid. The acidic fluids caused the soil to collapse rapidly by separating the anions and cations in clay interlayer. When the soil is contami- nated by acidic fluid, in the liquid phase a large proportion of the salt is dissolving and solid-phase property of the soil changed and it caused the reduction in soil strength and therefore the collapse happened.

The reason for the decrease in soil collapse at alkaline conditions (pH > 7) is related to the reduced solubility of ions. Therefore, most ions are remaining in solid form in soil and only a small change in collapse potential occurred.

Fig. 6 shows the collapse potential of soil in a saturated state with different chemical solutions.

4.3 Results of the direct shear test when samples were saturated with chemical solutions at different pH levels To determine the effective cohesion and effective internal friction angle parameters of soil, the results of the direct

shear test (vertical stress-shear stress) were shown in Fig. 7. It should be noted that the direct shear test was car- ried out on soil samples after the collapse condition hap- pens and each test was performed in three repetitions and the average of them was presented.

As shown in Fig. 7, the effective cohesion parameter and effective internal friction angle for soil saturated with water (pH = 7) were 14.5 kPa and 18.5°, respectively.

The tests were performed on the remolded samples which were saturated with chemical solutions at different pH levels. The results of the direct shear test on the satu- rated samples are shown in Table 6.

Fig. 5 Comparison of the behavior of collapse potential vs pH Fig. 6 Comparison of collapse potential in saturated conditions with different pH values

Fig. 7 Soil Failure envelope after saturation with water (pH = 7)

Table 6 Direct shear test results under saturated conditions with different pH values

pH C' (kPa) Ø'(˚)

3 35.33 16.04

5 30.86 17.25

7 14.5 18.5

9 11.34 20.53

11 8.16 21.89

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It can be seen in Table 6 that the effective cohesion parameter of the soil increased 144 % when the pH levels decreased from 7 to 3 and decreased 78 % when the alka- line solution is used as the fluid. In conclusion, the range of effective cohesion changes in acidic states were much greater than that of alkaline state. Compared to the previ- ous research, an increase in the cohesion of clayey soil in acidic condition was observed by Olgun and Yildiz [16], In addition Li et al. [21] reported a decrease in the cohe- sion of clayey soil in alkaline condition, which are similar to the results of the present study.

Therefore, it is observed that effective internal friction angle of the soil increased by increasing the pH levels and it was 16.04° and 21.89° when the pH level of the soil was 3 and 11, respectively. Also, it was proved that in acidic condition, the effective internal friction angle of the soil was in lowest. Fig. 8 shows the comparison pattern of rup- tures for samples when saturated with different pH levels.

To investigate the soil behavior better, normal tension has extended up to 1000 kPa in soil failure envelope.

Since in the direct shear test, the soil is under verti- cal stress, it was observed that in the acidic condition, the collapse of the soil increased. At the acidic condition, the soil became denser and showed higher shear strength.

Therefore, when the shear strength is high, the effective cohesion of the soil is high as well.

Similarly, according to Fig. 5 and the results of Table 6, it can be concluded that by increasing the pH of the solu- tion, the settlement of the test soil decreased and there- fore required a less horizontal force for the initial shear- ing. Note that with decreasing shear force, the rupture

diagram is placed at a lower altitude and effective cohe- sion decreased. Fig. 9 shows the percentage of soil shear strength variations in each pH compared to the shear strength of the soil at pH = 7.

As shown in Fig. 9, at lower pH, the shear strength of the soil increased due to an increase in effective cohesion. For example, at pH = 3, at first 144 % increase in shear strength at ϭ' = 0 kPa and 34 % increase in shear strength at ϭ' = 1 kPa was observed. But at the higher vertical stress, the shear strength of the soil decreased. Therefore, it can be concluded that due to the high values of vertical stress, by decreasing the pH of the solution and becoming more acidic, the effective internal friction angle decreased. According to Mohr Coulomb equation, the shear strength of the soil decreases even with increasing effective cohesion.

Similarly, at high pH, first, the shear strength of the soil is reduced then with increasing vertical stress, the shear strength of the soil increased, which increased the effective internal friction angle at high pH. According to the Mohr Coulomb stress equation, the increase in shear strength of the soil might happen even with decreasing effective cohe- sion. For example, at pH = 11 at first, 44 % decrease in shear strength at ϭ' = 0 was observed, but the vertical stress increased as the shear strength of the soil increased.

4.4 The results of unconfined compression test

Figs. 10 and 11 show the undrained shear strength ver- sus the axial strain for the contaminated soil samples which were contaminated at the different ratios of acid- ity (pH = 3) and alkalinity (pH = 11). It is observed that with increasing the acidity and alkalinity of the chemical

Fig. 8 Failure envelope graphs under saturated conditions with different pH values

Fig. 9 Percentage of soil shear strength variations in each pH compared to the shear strength of the soil at pH = 7

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solution, the undrained shear strength of the contaminated soils decreased, which led to a decrease in strength as the percentage of chemical solution in the soil increased.

The results are similar to previous study carried out by Yang et al. [18] concluding that in acidic and alkaline con- ditions, the unconfined compressive strength (UCS) of cementitious soils decreased.

4.5 X-ray diffraction (XRD) results

To better understand the effect of acidic and alkaline solutions on soil minerals, the XRD test was carried out on three soil samples (1) soil without contamination at pH = 7, (2) contaminated soil with chemical solution at pH = 3 and (3) contaminated soil with chemical solu- tion at pH = 11. The results of the XRD test are shown

in Fig. 12. It can be observed that the dominant miner- als of soil are quartz, albite, calcite, and montmorillonite.

It was shown that the soil phase and the ratio of miner- als have changed with the addition of acidic and alka- line solutions into the soil. Moreover, it was found that an increase in the percentage of quartz minerals in acidic and alkaline conditions occurred. The percentage of albite mineral was increased with increasing pH and was decreased in acidic conditions.

4.6 Scanning electron microscopy (SEM) results

To better understand the effect of acidic and alkaline solutions on soil structure, the SEM test was performed on three soil samples similar to the XRD test. The SEM images of three soil samples are presented in Fig. 13. It can be seen that the contaminated soil with pH = 3, still retained the pore structure with many void spaces. The change in the microstructure of minerals caused weaken the bond between the grains and as a result, the increasing collapse is justified. However, in contaminated soil with chemical solution at pH = 11, soil cavities have decreased, which has led to a reduction in soil collapse in alkaline condition.

5 Discussion

In this study, the effect of chemical solution at different pH levels on collapsible loess soil was studied. According to the fact that the soil studied was fine-grained and placed in the CL soil type, the results were almost similar to those of the previous studies. However, this study compared to the previous studies comprehensively investigated the effect of pH from acidic to alkaline level.

A more detailed look at the tests shows that significant changes in soil conditions occur with the addition of chem- ical solutions to the collapsible soil. Since the stability of soil structure and the strength of collapsible soil is directly related to particle bonding of the soil, the change in the soil phase and the ratio of minerals has a major impact on the conditions of the collapsible soil. SEM test shows changes in the microstructure of the soil which is visible in both acidic and alkaline conditions. On the other hand, the XRD test shows that the chemical composition of the soil has also changed. Based on the available evidences in SEM and XRD tests, results of the collapse, direct shear and UCS tests can be investigated.

According to the SEM test, cavities of collapsible soil increase in acidic conditions that cause an increase in col- lapse potential and a decrease in soil strength which is reasonably expected. But the opposite result is observed

Fig. 10 the influence of acidic solution on the undrained shear strength of collapsible soil

Fig. 11 The influence of alkaline solution on the undrained shear strength of collapsible soil

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in the contaminated soil with an alkaline chemical solu- tion, which led to a decrease in collapse potential and an increase in soil strength based on the SEM test.

As it was clearly explained in the sample preparation sec- tion, the condition of testing collapse potential and direct shear tests were quite different from the UCS tests. This shows that the behavior of soil may be significantly differ- ent according to the method of adding chemical solutions to the soil and one of the reasons for the different results of researchers is related to the condition of sampling.

6 Conclusions

In this paper, the effects of chemical solutions with differ- ent pH on the collapse potential, shear strength parame- ters and unconfined compression of collapsible soils were

investigated. The results of collapse tests showed that by decreasing the pH of the chemical solution from 7 to 3, the rate of collapse increased by 5.08 percent and with increas- ing pH from 7 to 11, the rate of soil collapse decreased by 2.04 percent that were proved in SEM images. Also, the range of changes in the collapse potential of soil was much higher in acidic conditions compared to alkaline conditions. The results of direct shear test showed that the effective cohesion increased from 14.5 kPa to 35.33 kPa in acidic condition and decreased from 14.5 kPa to 8.16 kPa in alkaline condition. It is also observed that the range of effective cohesion parameters in acidic condition was much higher than alkaline condition. It was also observed that effective internal friction angle of soil decreased by 2.46° in acidic condition and increased by 3.39° in

Fig. 12 The results of XRD tests for samples contaminated with chemical solutions at different pH levels

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Fig. 13 The results of SEM tests for remolded loess soils (a) Clean soil (b) Contaminated soil with chemical solution at pH = 3 (c) Contaminated soil with chemical solution at pH = 11

alkaline condition. The results of unconfined compres- sion test showed that with increasing the acidity and alka- linity in chemical solution, the undrained strength of the soil decreased. Consequently, in the case of acidic con- tamination, the risk of an increase in collapse potential should be considered. While alkaline pollutants do not

only cause any concern for the problem of collapse, but alkaline materials also can be used to some extent to reduce the collapse potential.

Conflict of interest

The authors declare that they have no conflict of interest.

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